where the middle map is the evaluation map [1]}. In particular, setting \(N=R\) , we see that the canonical
map \(M\rightarrow M^{**}\)  is part of the exact sequence
\(0\rightarrow \operatorname{Ext}^1_R(\operatorname{Tr}M,R)\rightarrow M\rightarrow M^{**}\rightarrow \operatorname{Ext}^2_R(\operatorname{Tr}M,R)\rightarrow 0.\qquad \mathrm {(\ref {a1}.3)}\) 
